In the search for methods of utilizing the sun's energy, a wide variety of solar collectors have been developed, including electrical conversion collectors, passive thermal collectors such as a dark-colored interior rock wall behind a pane of glass, concentrating thermal collectors utilizing either a double convex lens or a parabolic mirror to either refract or reflect direct radiation to a specific point, and flat plate thermal collectors.
Because of the many practical problems associated with the others, most research has been devoted to the flat plate thermal collector, which is available in a variety of designs. The essential components of a flat plate collector are a transparent sheet such as glass or plastic, commonly referred to as glazing, which allows shortwave radiation from the sun to pass but is impervious to longwave radiation, a dark-colored, generally opaque energy-absorbing surface to absorb the shortwave solar radiation and convert it to longwave radiation, and a convection heat transfer medium, such as air or water.
The principal drawback of most prior art flat plate collectors has been that they are not efficient at capturing two of the three available types of usable light energy, i.e., indirect and diffuse radiation, and lose substantial amounts of the third type of light energy, i.e., direct radiation, when the angle of light incidence is fairly large, as in the early morning or late afternoon. Such losses occur both through reflection from the planar energy-absorbing surface and as a result of a decrease in the effective area of exposure whenever the angle of incidence of the impinging light is great. Still another drawback of flat plate collectors is the requirement that they be substantial in size and installed south to southwest at an angle equal to the sum of the latitude of its location plus 15.degree. in order to accomplish maximum collection of thermal energy, necessitating expensive structural support.
One prior art improvement on the flat plate collector has featured the use of perpendicular vanes in connection with the energy-absorbing surface, accomplished by the use of either a large number of aluminum cups or L-shaped plates supported upon a planar surface and closed at their bases, as set forth in U.S. Pat. Nos. 3,894,685, 3,946,720, 3,946,721 and 4,088,266. While improved flat plate collectors of this type are effective to obviate many of the energy-collecting deficiencies of other flat plate collectors described in the previous paragraph, they do little to enhance, and may even diminish, the convection heat transfer characteristics of the collector. Since the heat transfer efficiency is as important as the energy-collecting efficiency of a flat plate collector, enhancing one without also enhancing the other is self-defeating. Thus, in the case of a collector wherein the collecting efficiency is enhanced by the use of cups closed at their bases, and particularly where the base closure constitutes the energy-absorbing surface, the impediment to flow of the convection medium, such as air, over the interior surfaces of the cups, and over the primary energy-absorbing surface, tends to inhibit rather than enhance the transfer of heat from the surfaces to the convection medium.
Accordingly, what is needed is a flat plate collector which is more efficient at capturing and collecting all three types of usable light energy while, at the same time, having correspondingly enhanced efficiency with respect to the transfer of heat by convection. It would also be desirable if such enhanced heat transfer efficiency could be obtained without requiring forced convection, such as by the use of a fan, since such forced convection systems themselves use significant amounts of energy.